Day-night rear view mirrors are well known for use in motor vehicles to allow an operator to selectively adjust the reflectivity of the mirror between high light conditions and low light conditions, e.g. to reduce the reflected headlight glare from following vehicles.
Typically, the mirror assembly is adjustable by the operator to select between a highly light-reflective surface for use during periods of daylight and a relatively low reflective surface for use during periods of low light, e.g. at night. The relatively low reflectivity minimizes glare perceived by the eyes of a vehicle operator from headlights of a following vehicle, which can be bothersome and also create an unsafe driving condition.
Many mirror assembly designs have been utilized to produce the day-night mirror effect. One such day-night rear view mirror is described in my U.S. Pat. No. 5,028,120, issued Jul. 21, 1991, entitled "Day-Night Rear View Mirror", the disclosure of which is incorporated herein by reference.
Another prior art design used extensively has a mirror with a triangular cross-section for producing two different images. Since about five percent of light rays normal to a glass surface are reflected, the image produced by that surface has a character appropriate for use as a rear view mirror in low light conditions. The remaining ninety-five percent of light rays pass through the surface of the glass to be reflected by the silvered backing of the mirror. Since the mirror has a triangular cross-section, rays of the image produced by the front surface of the glass diverge from rays of the image produced by the silvered back of the glass. By the same token, images from two different points or sources of light converge to a single point, i.e. at the motor vehicle operator. When the mirror assembly is mounted, e.g. in an automobile, the roof of the vehicle is the point source for one image. Since there is no light source in the roof of the vehicle, only one image is visible to the operator. Therefore, by adjusting the position or tilt of the mirror, the operator can selectively view either an image from light reflected from the glass surface, or an image produced by light passing through the glass surface to be reflected by the silvered back of the glass.
Unfortunately, a mirror assembly utilizing a movable triangular mirror or prism is limited to use within a vehicle. If used on the exterior of a vehicle, e.g. a truck or motorcycle, or in a convertible with the top down, two images are visible to the operator simultaneously. For example, as the operator drives down a street lit by street lights with cars following behind, the street light images are viewed by the operator as overlapping the image(s) of car headlights behind the operator.
Others have sought to address this problem with a day-night rear view mirror having a transparent glass panel positioned within a housing between a mirror and an area to be viewed. The mirror is surrounded by a movable opaque shade with a window that is brought into and out of registration with the light reflective surface of the mirror. For example, for night viewing, the window is placed out of registration, with the mirror covered by the opaque shade material, so that images such as headlights of following vehicles are reflected by the plain glass panel in front of the mirror. Since the light reflectivity of the plain glass panel is much lower than the light reflectivity of the mirror, the reflected image viewed by the vehicle operator has greatly diminished intensity, i.e. glare. For daylight use, the window is placed in registration with the mirror so that images from the rear of the vehicle pass through the transparent glass panel to be reflected by the mirror. This day-night rear view mirror designed for mounting outside a vehicle has also often been found less than satisfactory in actual use. For example, during daytime viewing, the operator can perceive a faint, second image offset from the principle image reflected by the mirror. This is an annoying condition that can distract the operator.